Knowing the state of fish stocks is the basis for determining TACs (Total Allowable Catches). However, there are other management measures that also require scientific advice.
Today we are talking about the Landing Obligation (LO)

@EvaUve

In the latest reform of the Common Fisheries Policy (CFP, 2013), the obligation to land (and therefore the prohibition to discard) was established any species subject to:
TAC and quotas (Atlantic)
Minimum Conservation Reference Sizes (MCRS) (Mediterranean)

With the aim of gradually reducing discards (catches that were previously returned to the sea for different reasons) the LO was gradually implemented in the different fisheries, being fully in force since January 1, 2019
@xulioval

There are some exemptions to LO:
Prohibited species
Species with a high survival rate (e.g. rays, Norway lobster)
De minimis exemptions *
Fish with damage caused by predators

* The European Commission may grant "de minimis exemptions" to certain metiers when it is scientifically proven that it is not easy to improve the selectivity of said metier they can exceptionally discard% of the species

The catches returned to the sea based on some exemption must be correctly recorded, which translates into an increase in the categories of the catch.
Before, only discarded species (DIS) and landed species (LAN) were discussed

Now, in addition, there are:
BMS (below minimum size): is the fraction of the catch that, being below the legal minimum landing size, must necessarily be landed
The category of "minimis"